1. Field of the Invention
The present invention relates to a transferer and an image forming apparatus, and more particularly to prevention of backside contamination of transfer materials after toner images are transferred onto the transfer materials.
2. Discussion of the Background
As commonly known, in image forming apparatuses such as copiers, facsimiles and printers, an electrostatic latent image formed on a photoreceptor as an image bearer is visualized with a developer fed from an image developer.
The visualized toner image is melted, penetrated and fixed on a transfer material to form a duplicate image thereon by a fixer upon application of heat and pressure after transferred onto a transfer material.
The duplicate image includes not only single-colored images such as monochrome images, but also multi-colored images including full-color images.
As image forming apparatuses for forming multi-colored images, particularly full-color images, tandem image forming apparatuses including image forming stations for each color located along an intermediate transfer belt as a first transferer in parallel, onto which images formed by the image forming stations are sequentially transferred are known.
The sequentially-transferred and overlapped images on the intermediate transfer belt are transferred by a second transferer onto a transfer material at a time, and the transfer material is transferred to a fixer.
Japanese published unexamined application No. 2007-292904 discloses a tandem image forming apparatus feeding the transfer material after the second transfer to a fixer with a feeding guide in a route from the second transfer site to the fixer.
A toner remaining on the background of the intermediate transfer belt occasionally transfers and adheres to the surface of a roller used as a member performing the second transfer. The toner adhering to the surface of the roller causes backside contamination of the transfer material. When the transfer material and the intermediate transfer belt are cleaned, the toner is occasionally discharged to easily clean them. However, some low-cost image forming apparatuses do not have such system and a toner inevitably adheres to the second transferer in many cases.
The toner adhering to the backside of a transfer material occasionally transfers to a guide face of the feeding guide located in the route to the fixer in the process of traveling thereto. A toner is occasionally peeled off when scraped by the guide face and does not adhere thereto. However, a toner is dragged thereby and occasionally stays at an edge face thereof toward the fixer.
When the toner accumulates too much at an edge face, a transfer material contacts the toner and the backside thereof is possibly contaminated.
In order to prevent the backside contamination of a transfer material caused by a toner accumulating near the edge of the feeding guide, Japanese published unexamined application No. 09-188439 discloses a feeding guide having a guide face including a rib having a height capable of forming a step as a space accumulating a toner at the edge. This avoids a contact between the toner and the transfer material by feeding the transfer material while floating the transfer material such that the end thereof does not enter the step, taking advantage of its stiffness.
In order to avoid a contact between the transfer material and the feeding guide, Japanese patent No. 3444778 discloses a feeding guide having a guide face including a back-and-forth movable swinging member, feeding a transfer material while setting the transfer material apart from the guide face. Alternatively, Japanese published unexamined application No. 11-172972 discloses a feeding guide having a spur floating a transfer material.
In order to prevent the backside contamination of a transfer material, as disclosed in Japanese published unexamined application No. 09-188439, a step is formed between a rib formed on the guide face and the guide face such that the transfer material is difficult to contact a toner accumulated in the step by a property of the transfer material itself, which is different from application of an outer force. This possibly cannot be expected to prevent the backside contamination without fail. Before explaining the reason, a generating mechanism of the backside contamination will be explained.
In FIG. 12A, a toner T adhering to a second transfer roller A transfers to the backside of a transfer paper S as a transfer material passing the second transfer position, and adheres thereto.
In FIG. 12B, when the transfer paper S travels along the guide face of a transfer exit-side guide member B, the toner adhering to the backside of a transfer paper S is dragged by a friction force between the transfer paper S and the guide face. When the transfer paper S reaches the edge of the guide member B, the toner T is not dragged thereby and accumulates at the edge as shown in an enlarged view in FIG. 12B. As a result, when the toner accumulates too much at an edge face, a transfer material contacts the toner and the backside thereof is possibly contaminated.
In Japanese published unexamined application No. 09-188439, a rib is used as a catapult for floating the edge of a transfer material just not to lead the end of the transfer material in the step as little as possible. Therefore, depending on the stiffness, i.e., elasticity or the weight of the transfer material, the transfer material possibly travels toward the step or a posterior portion thereof possibly contacts to an edge of the step when bending toward the guide face. There still is a possibility that a transfer material contacts a toner accumulating on the step and the backside thereof is contaminated.
The present inventor made an experiment based on Japanese published unexamined application No. 09-188439 only to find that the backside of a transfer material is still contaminated.
As shown in FIG. 13A, a step is formed with a rib on a guide face of the transfer exit-side guide and an experiment was made under the following conditions to see generation of the backside contamination.
(1) Fifty both-sided images having a high image area were produced to contaminate the guide.
(2) One hundred one-sided images were produced to count the backside contamination.
(3) A transparent adhesive tape was put on the whole guide to see whether a toner stays on a guide plate.
As a result, 48 out of 100 images had backside contamination. The transparent adhesive tape was contaminated with a toner. A shown in FIG. 13B, a position having a backside contamination was just shifted and did not disappear, and the backside contamination having the shape of a horizontal stripe remained.
With only a feeding guide having a guide face including a step in a feeding route for a transfer material, a toner cannot be expected to naturally fall from the step by gravity because the step does not have a vertical face and is likely to accumulate therein. Therefore, the accumulated toner is likely to contact a transfer material.
Because of these reasons, a need exists for a transferer reliably preventing the backside contamination of a transfer material.